Although a number of compounds have been experimented with to measure free chlorine, the measurement of free chlorine in water involving the formation or loss of coloured species
has long been determined by three principal methods:                (1) titrimetry where free chlorine stoichiometrically reacted with a reducing agent such as ferrous ion to an end-point utilising a redox indicator such as N,N-Diethyl-p-phenylenediamine (DPD).        (2) colorimetry where an excess of a colourless reagent reacts with free chlorine to produce a coloured reaction product proportional to the concentration of free chlorine. For example, DPD reacts with free chlorine to produce a pink colour.        (3) Bleaching of methyl orange.        
Reactions involving production or loss of colour can be monitored accurately by measuring absorption at one or more wavelengths.
Method (1) has the disadvantage that a dilute solution of ferrous ion is readily oxidised by dissolved oxygen. This necessitates the use of fresh titrant. Moreover, at the commonly measured concentrations of free chlorine between 0-5 ppm, the end-point is not sharp.
DPD, the most popular reagent of Method (2) is injurious to health. The solid may be fatal if absorbed through the skin (ref: MSDS Oxford University). This risk is reduced for DPD in solution, but poses a concern where an on-line swimming pool or spa analyser utilises a reservoir of DPD. If the analyser malfunctions and a significant amount of DPD is discharged into the water, the health of a bather could be affected, especially in the case of a small body of water, as in a spa.
Methyl orange is also injurious to health. Bleaching methods can potentially measure a larger range of free chlorine concentrations, but enough of the compound being bleached must be added to ensure all of the free chlorine present reacts. It is also necessary to know the volume which has been added in order to calculate the concentration of free chlorine.
As on-line analysers using colorimetric methods consume part of the body of water they measure, this may result in a significant loss of water over time if the reagent/water mixture is expelled to waste. This is undesirable in areas where water conservation is paramount. Moreover, consideration must be given to the safe disposal of the spent reagent/sample mixtures.
Many of the methods of the prior art involve several separate reagents which are dispensed from separate reagent reservoirs. This requires complex and expensive measuring apparatus For example the Standard Method for the measurement of free chlorine involves separate solutions of DPD, and buffer.
European patent 762120 discloses a method of determining chlorine content by using a benzidine indicator which forms a dye by reaction with chlorine.
U.S. Pat. No. 4,092,115 discloses using an azine indicator for chlorine analysis.
U.S. Pat. No. 7,441,472 discloses a method and device for measuring chlorine content in swimming pools.
U.S. Pat. No. 7,491,546 discloses a touch free method of chlorine analysis.
WO2008061315 by the present applicant discloses apparatus and method for determining chlorine content is a swimming pool.
It is an object of the present invention to provide a reagent for the measurement of free chlorine which is not injurious to health, and may be safely discharged into water used for drinking or swimming purposes, thereby allowing for the recycling of spent reagent/sample mixtures of free chlorine back into the body of water being measured, in order to conserve water.
It is preferred object of the present invention to provide a control system in order to regulate the addition of a reagent to provide a simple, accurate and reliable addition of said reagent to a fixed volume of sample fluid to be measured.
It is another preferred object of the invention to provide a concentrated reagent to minimise reagent consumption and so that the addition of the reagent to the fixed volume of sample causes a negligible dilution of the sample. It is also an object of the invention to use single reagents within which all the required components have a stable coexistence to simplify the dispensing system, and to increase the speed of the method by using a single injected volume as opposed to a titrimetry process where multiple injections are required.
Yet another preferred object of the invention is to allow accurate measurements of free chlorine over a wide range of concentration without need for dilution of the sample, typically 0-25 ppm.